This invention relates to a method of making an aqueous solution of potassium sulfite which has an APHA number of less than 40. In particular, it relates to manufacturing potassium sulfite from potassium hydroxide which contains a low concentration of iron, either because it was selected for low iron or because the iron was inactivated with hypophosphorous acid.
Potassium sulfite is manufactured by reacting potassium hydroxide in solution with liquid sulfur dioxide. The resulting solution of potassium sulfite is sometimes clear and is sometimes yellow or brownish in color. Until now, the cause of this yellow or brownish color was unknown.
Potassium sulfite is sold for use in a variety of applications. One application is in a developing solution for photographic negatives. While potassium sulfite that has the yellow or brownish color does not result in lower quality pictures, nevertheless, customers who use developing solutions prefer clear solutions and will purchase a clear developer in preference to a colored developer. In order to be acceptable for use as a developing solution the potassium sulfite solution should have an APHA (American Public Health Association) number that does not exceed 40 and is preferably less than or equal to 10 (based on a 45 wt % solution of K.sub.2 SO.sub.3).
In order to satisfy the customers without discarding or wasting yellow or brownish potassium sulfite, it has been the practice in the industry to segregate high color potassium sulfite for customers which have applications not related to photographic developing (e.g., chlorine scrubbing solutions).